When the environment presents a selective force, such as a new predator, natural selection is the primary force by which populations adapt.
Consider this original population as an example, which consists of beetles varying in color. Evolution by natural selection requires that a population have genetic variation. That is, individuals possess inherited differences.
Individuals with coloration traits that make them stand out visually are more likely to be eaten. Evolution by natural selection occurs when the population shows differential reproductive success; in this example, not everyone survives to reproduce.
Because survivors pass on those adaptations to their offspring, the gene frequencies of the population change in the next generation, which means some traits are more common and others are less common than they used to be. When this happens, the population is said to have evolved.
Over time, the beetle population may be mostly or solely made up of tan individuals.
Natural selection can have different outcomes, depending on what varieties in the population the environment favors or selects against. This graph represents the original population of beetles. The population has mostly tan beetles, with fewer light and dark beetles.
Consider three scenarios, in which each population starts out with the same distribution of beetles. One population lives on trees that are tan. Another lives in areas with trees darkened from pollution, and another lives in a forest with both light and dark trees, but no tan trees.
As you can imagine, the forest with tan trees would provide better camouflage for the tan beetles, whereas the darker and lighter trees would provide better camouflage for darker and lighter beetles.
Over the generations, the beetles with the less advantageous traits will be selected against. Over the generations, the population will evolve to have relatively more individuals with the advantageous traits and fewer individuals with the traits selected against.
Several different kinds of selection have occurred. Stabilizing selection favors the norm and selects against the extremes. Directional selection continually favors a particular extreme of the trait. Disruptive selection favors the extremes but selects against the intermediate forms.